Magnetic resonance imaging (MRI) is a technique in which an object is placed in a magnetic field and subjected to pulses of the electromagnetic field at a frequency. The pulses cause nuclear magnetic resonance in the object, the spectra obtained thereby being processed numerically to form cross-sectional images of the object. MRI imaging is especially useful for medical or veterinary applications because different living tissues emit different characteristics of resonance signals, thus enabling visualization of the different living tissues in the obtained image. An MRI apparatus thus operates in general by the application of a radio frequency (RF) electromagnetic field in the presence of other magnetic fields, and the subsequent sensing and analysis of the resulting nuclear magnetic resonances induced in the body.
Conventional MRI systems include a main magnet which generates a strong static magnetic field of a high temporal stability and a high spatial homogeneity within a field-of-view (FOV) where the imaging takes place. Conventional MRI systems also include a gradient coil assembly located in the bore between the main magnet and the RF coil and generating space-varying fields. The gradient coil assembly causes the response frequency and phase of the nuclei of the patient body to depend upon position within the FOV thus providing a spatial encoding of the body-emitted signal. Conventional MRI systems further include RF coil/coils arranged within the bore which emit RF waves and receive resonance signal from the body. The superconducting main magnet is typically used to achieve high field strength; superconducting main magnet comprises a plurality of concentric coils placed inside a cryostat which is designed to provide a low temperature operating environment for superconducting coils.
The confined space in the bore of MRI scanners often causes patient discomfort and a feeling of claustrophobia. In an attempt to reduce the discomfort of the patients and claustrophobic feelings, magnet designers typically try to minimize or shorten the length of the magnet. Unfortunately, reducing the length of the magnet also reduces the length of the uniform field region and compromises the imaging functionality. Short magnets typically have symmetrical, ellipsoidal FOVs, compared to spherical FOVs on longer magnets.
FOV requirements 900 for various image types are illustrated FIG. 9 where it can be seen that many scans require a FOV length of 40 centimeters (cm). Whole spine and peripheral vascular studies require an even longer uniform field region of approximately 45 cm-50 cm length.
As shown in FIG. 9, MRI systems with shortened FOVs do not accommodate imaging of anatomy that requires a large FOV, such as imaging of a whole spine or imaging of peripheral vascular studies.
Conventional solutions to overcome the effects of the shortened FOV include the use of a multiple scan system, where the patient is physically moved along the bore between scans. However the multiple scan system suffers from increased scan duration and mechanical complexity which is expensive to manufacture and maintain.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a magnetic resonance imaging system that is more comfortable for patients, is less likely to invoke a sense of claustrophobia in patients, does not reduce the length of the uniform field region, and does not compromise the imaging functionality without adding mechanical complexity.